This invention relates to a method of controlling the engine rpm of an internal combustion engine, and more particularly to a method of this kind which is capable of improving the accuracy of control of the engine rpm, by minimizing fluctuations in the engine rpm due to changes in the electrical loads applied on the engine during rpm control of the engine.
An idling rpm feedback control method is known e.g. from Japanese Patent Provisional Publication No. 55-98628, wherein, after setting the desired idling rpm in response to the engine load at the time of engine idle, supplementary air is supplied to the engine in an amount depending on the difference between the desired idling rpm and the actual engine rpm, so as to make this difference zero to thereby bring the engine rpm to the desired idling rpm. During idling rpm feedback control (hereinafter called "feedback mode control"), according to the above known method, if electrical devices such as head lamps, an air conditioner, etc. are operated, such operations result in increasing the engine load. Further, in order to recharge the battery to supply power to these electrical devices when the battery output voltage drops below a certain level, the generator has to function, resulting in increased engine load and a consequent drop in the engine rpm. Even in the case of such a drop in the engine rpm, the engine rpm recovers a value close to the desired idling rpm by the feedback mode control. But, in case such electrical load applied on the engine is very large, it can result in engine stall, or if the vehicle is started simultaneously with the addition of such electrical load, the clutch can not be operated smoothly.
It is also a well known control method to control the rpm of the engine by gradually increasing the quantity of supplementary air being supplied to the engine with a decrease in the engine rpm from the time the engine rpm reaches a level below a predetermined rpm to the time it reaches the upper limit of the desired idling rpm range, while the engine is decelerating with its throttle valve fully closed, in order to prevent engine stall caused by a sudden drop in the engine rpm due to disengagement of the clutch (e.g. Japanese Patent Provision Publication No. 55-98629). Even in the case of such control of the supply of supplementary air to the engine during deceleration (hereinafter called "deceleration mode control"), if electrical load is added particularly when the clutch is in a state of disengagement, the engine speed can abruptly drop, resulting in engine stall, as the quantity of supplementary air then supplied becomes insufficient.
Further, in the event the engine is accelerated with the throttle valve opened, during feedback mode control of the idling engine, if the supply of supplementary air to the engine is interrupted simultaneously with the opening of the throttle valve the total intake air quantity being supplied to the engine abruptly drops, causing a drop in the engine rpm and making it difficult to engage the clutch without causing engine stall.
In order to eliminate this difficulty, it has been proposed by the applicant of the present application that the quantity of supplementary air to be supplied immediately after the opening of the throttle valve is set to the quantity of supplementary air determined during feedback control mode immediately before the opening of the throttle valve, and this quantity of supplementary air is then gradually decreased in proportion to the increase in the engine rpm. During this gradual decrease of the supply of supplementary air (hereinafter called "acceleration mode control"), if electrical load is added to the engine load, the engine speed abruptly drops, causing discomfort to the driver, because at this stage, the engine rpm is not yet large enough and the engine output is also not sufficient.